My computer (24" 3.06Ghz intel core 2 duo) won't start. When I try to turn on or restart my iMac I get vertical rows of pixelated lines across my monitor on the white startup background with the apple logo and it will either freeze up or the background turns gray and I get a message that says I should restart my computer. This has also happened when restarting using the Apple hardware test (which doesn't report any errors). My warranty has expired so what should I do? My computer problem looks similar to this image.